Compositions of silico-sodo-calcic glasses and their applications

ABSTRACT

This invention concerns a thermally stable glass composition, which comprises the following constituents in the following proportions by weight: 
     
         ______________________________________                                    
 
    
            SiO 2      45 to 68%                                            
       Al 2  O 3                                                    
                      0 to 20%                                            
       ZrO 2       0 to 20%                                            
       B 2  O 3                                                     
                      0 to 10%                                            
       Na 2  O     2 to 12%                                            
       K 2  O    3.5 to 9%                                             
       CaO            1 to 13%                                            
       MgO            0 to 8%                                             
______________________________________                                    
 
     the sum of the contents of the oxides SiO 2 , Al 2  O 3  and ZrO 2  remaining equal to or less than 70%, the sum of the oxides Al 2  O 3  and ZrO 2  being equal to or greater than 2%, the sum of the alkaline oxides Na 2  O and K 2  O being equal to or greater than 8%, said composition comprising, if desired, the oxides BaO and/or SrO in proportions such that: 
     
         11%≦MgO+CaO+BaO+SrO&lt;30% 
    
     This glass composition may be used for the production of substrates for an emissive screen or for that of a fire-resistant pane.

This invention has as its subject glass compositions suitable for beingconverted into glass ribbon, from which can be cut plates that arehighly resistant to heat. These plates may be used for the production offire-resistant panes or may serve as substrates for the production ofplasma screens, electroluminescent screens and cold cathode screens(field-emission displays).

The glass at present used for the production of such substrates is aglass belonging to the family of the silico-sodo-calcic glasses,commonly used for the production of panes intended for buildings orautomobile vehicles. Although this type of glass is thoroughlysatisfactory with regard to its chemical resistance, planeity and thedefects that it does have, its temperature behaviour sometimes leavessomething to be desired.

In the production of emissive screens, the substrate is subjected toseveral thermal treatments which are intended for stabilizing thedimensions of said substrate and fixing a series of films of differentcompounds, such as enamels, deposited on its surface. The fixing ofthese films of more or less large thicknesses requires that thetemperature of the substrate be raised to values higher than 550° C.Although the coefficient of thermal expansion of the silico-sodo-calcicglass used may be of the same order of value as that of the compoundsdeposited on its surface, its temperature behaviour is inadequate and itmust be supported on a trued plate during the thermal treatments toprevent any deformation.

The glasses used for the manufacture of fire-resistant panes generallybelong to the family of the borosilicate glasses. These glasses, whichhave very good heat resistance and thermal shock resistance arecharacterized generally by a low coefficient of thermal expansion. Thislatter characteristic does not allow high stresses to be developed inthese glasses by thermal toughening and the increase of their mechanicalstrength by this means is accordingly limited.

The present invention proposes to overcome the limitations that the useof these known glasses imposes for one or another of the applicationsindicated above.

Thus, the present invention has as its subject a glass compositionenabling a plate or a substrate to be produced, the deformation of whichis virtually zero when it is subjected to temperatures between 550° and600° C.

The present invention has as its subject, notably, a glass compositionenabling a plate to be produced, within which stresses at least as highas those that develop in a plate of ordinary silico-sodo-calcic glasscan become established by thermal toughening.

The present invention has as its subject a glass composition enabling asubstrate to be produced, in which the surface depletion of alkalineions shall be less than that observed on a substrate produced from anordinary silico-sodo-calcic glass.

The present invention also has as its subject a glass compositioncapable of being melted and converted into a ribbon of float glass on amolten metal bath in temperature conditions close to those of anordinary silico-sodo-calcic glass.

These objectives are attained by means of a glass composition whichcomprises the following constituents, in the following proportions byweight:

    ______________________________________                                               SiO.sub.2     45 to 68%                                                       Al.sub.2 O.sub.3                                                                             0 to 20%                                                       ZrO.sub.2      0 to 20%                                                       B.sub.2 O.sub.3                                                                              0 to 10%                                                       Na.sub.2 O     2 to 12%                                                       K.sub.2 O    3.5 to 9%                                                        CaO            1 to 13%                                                       MgO            0 to 8%                                                 ______________________________________                                    

the sum of the contents of the oxides SiO₂, Al₂ O₃ and ZrO₂ remainingequal to or less than 70%, the sum of the oxides Al₂ O₃ and ZrO₂ beingequal to or greater than 2%, the sum of the contents of the alkalineoxides Na₂ O and K₂ O being equal to or greater than 8%, saidcomposition comprising, if desired, the oxides BaO and/or SrO inproportions such that:

    11%≦MgO+CaO+BaO+SrO≦30%

said composition having a strain point equal to or greater thanapproximately 530° C. and a coefficient of expansion (α₂₅°-300° C.)lying between 80 and 95×10⁻⁷ /°C.

It is commonly accepted that glass no longer has any viscous behaviourbelow a characteristic temperature termed the strain point, whichcorresponds to a viscosity of the order of 10¹⁴.5 poises. For thisreason, this temperature is an interesting reference point forevaluating the behaviour with temperature of a glass. As a result of thecombination of the constituents as it results from the definition ofthis invention, the glasses complying with this definition possess astrain point higher by at least 25° C. approximately than that of aconventional silico-sodo-calcic glass. For the majority of the glassesaccording to the invention, this difference is at least 45° to 50° C.

This combination of constituents also enables glasses to be obtained,the coefficient of thermal expansion of which remains of the same orderof value as that of a conventional silico-sodo-calcic glass.

The glasses according to this invention also have the advantage that itis possible to melt them and convert them into a glass ribbon attemperatures in the vicinity of those adopted for the production of aconventional silico-sodo-calcic glass.

In this respect, SiO₂ fulfils an essential role. In the context of theinvention, the SiO₂ content must not exceed approximately 68%; abovethis value the melting of the vitrifiable mixture and the refining ofthe glass require high temperatures, which cause accelerated wear of therefractories of the furnaces. Moreover, it has been observed in theframework of this invention that an increase in the SiO₂ content doesnot favour a rise in the strain point of the glass. Below 45% by weightof SiO₂, the stability of the glasses according to the invention isinsufficient. The glasses according to the invention, which most easilymelt, the viscosities of which are best adapted to the floating of theglass on a bath of molten metals and which have the highest strainpoints, contain between 45 and 59% of SiO₂.

Alumina fulfils a stabilizing function. This oxide increases to acertain extent the chemical resistance of the glass and favours theraising of the strain point. The percentage of Al₂ O₃ should not exceed20%, since otherwise there is a risk that melting may be made toodifficult and that the viscosity of the glass at high temperatures maybe increased to an unacceptable extent.

Zro₂ also fulfils a stabilizing role. This oxide increases, to a certainextent, the chemical resistance of the glass and promotes a rise in thestrain point. The percentage of ZrO₂ should not exceed 20% sinceotherwise there is a risk that melting may be made too difficult. Ifthis oxide is difficult to melt, it does have the advantage of notincreasing the viscosity of the glasses according to this invention athigh temperatures. This makes it possible to avoid the introduction,into these glasses, of oxides such as B₂ O₃, one of the effects of whichis to reduce the viscosity of the glass, or to avoid increasing thecontent of alkaline oxides, which have the same effect.

Overall, the melting of the glasses according to this invention remainswithin acceptable temperature limits, provided that the sun of thecontents of the oxides SiO₂, Al₂ O₃ and ZrO₂ remains equal to or lessthan 70%. The term acceptable limits is to be understood as meaning thatthe temperature of the glass corresponding to logη1.6 does not exceedapproximately 1,630° C. and preferably 1,590° C.

Among the glasses of the present invention, some contain alumina and,possibly also, zirconia, and others contain zirconia, and possiblyalumina. To distinguish these in the remainder of the description, theformer will be termed aluminous and the latter zirconiferous.

The glasses termed aluminous contain the following constituents in thefollowing proportions by weight:

    ______________________________________                                               SiO.sub.2     45 to 68%                                                       Al.sub.2 O.sub.3                                                                             2 to 20%                                                       ZrO.sub.2      0 to 20%                                                       B.sub.2 O.sub.3                                                                            0.5 to 4%                                                        Na.sub.2 O     4 to 11%                                                       K.sub.2 O    3.5 to 7%                                                        CaO            1 to 13%                                                       MgO            1 to 8%                                                 ______________________________________                                    

the sum of the contents of the oxides SiO₂, Al₂ O₃ and ZrO₂ remainingequal to or less than 70%, the sum of the contents of the alkalineoxides Na₂ O and K₂ O being equal to or higher than 8%, said compositioncontaining, possibly also, the oxides BaO and/or SrO in proportions suchthat:

    11%≦MgO+CaO+BaO+SrO≦24%

This family of glasses is distinguished, notably by the systematicpresence of B₂ O₃. In fact this oxide, as lattice-forming oxide, may beadded to or may replace SiO₂. It lowers the fusion point of thevitrifiable mixture as well as the viscosity of the glasses at hightemperatures. It also reduces the tendency of the glass to devitrify,and in particular it enables a rise in the liquidus temperature to beprevented. This effect, combined with the lowering of the viscosity,enables a sufficient difference to be maintained between the mouldingtemperature of the glass and its liquidus temperature. In float glasstechnology, in particular, it is important for the liquidus temperatureof the glass to remain equal to or lower than the temperaturecorresponding to logη=3.5, which is the case with the glasses accordingto this invention. More specifically, the glasses according to theinvention have a temperature corresponding to logη=3.5 that is equal toor lower than approximately 1,220° C. and preferably 1,170° C.

The B₂ O₃ content does not exceed approximately 4% because, above thisvalue, the volatilization of boron in the presence of alkaline oxidesduring the production of the glass can become significant. In thisfamily of glasses according to the invention, the sum of the contents ofthe oxides Al₂ O₃ and ZrO₂ is advantageously equal to or greater than5%. The sum of the contents of these oxides is, preferably, between 8and 22%.

The preferred compositions of aluminous glass according to the inventioncomprise the constituents given below in the following proportions byweight:

    ______________________________________                                               SiO.sub.2                                                                           45 to 59%                                                               Al.sub.2 O.sub.3                                                                    5 to 18%                                                                ZrO.sub.2                                                                           0 to 17%                                                                B.sub.2 O.sub.3                                                                     0.5 to 4%                                                               Na.sub.2 O                                                                          4 to 10%                                                                K.sub.2 O                                                                           3.5 to 7%                                                               CaO   1 to 12%                                                                MgO   1 to 7%                                                          ______________________________________                                    

the sum of the contents of the oxides SiO₂, Al₂ O₃ and ZrO₂ remaininglower than or equal to 70%, the sum of the contents of the alkalineoxides being equal to or greater than 10%, said compositions comprising,if desired, the oxides BaO and/or SrO in proportions such that:

    14%≦MgO+CaO+BaO+SrO≦22%

said glass compositions having a strain point equal to or higher than550° C. and a coefficient of thermal expansion (α₂₅°-300° C.) lyingbetween 85 and 95×10⁻⁷ /°C.

The glasses termed zirconiferous comprise the constituents listed belowin the following proportions by weight:

    ______________________________________                                               SiO.sub.2                                                                           45 to 63%                                                               ZrO.sub.2                                                                           6.5 to 20%                                                              Al.sub.2 O.sub.3                                                                    0 to 18%                                                                Na.sub.2 O                                                                          4 to 12%                                                                K.sub.2 O                                                                           3.5 to 7%                                                               CaO   1 to 13%                                                                MgO   1 to 8%                                                          ______________________________________                                    

the sum of the contents of the oxides SiO₂, ZrO₂ and Al₂ O₃ remainingequal to or less than 70%, the sum of the contents of the alkalineoxides Na₂ O and K₂ O being equal to or greater than 8%, saidcomposition containing, if desired, the oxides BaO and/or SrO inproportions such that

    11%≦MgO+CaO+BaO+SrO≦24%

said composition having a strain point equal to or higher thanapproximately 530° C. and a coefficient of thermal expansion (α₂₅°-300°C.) lying between 80 and 95×10⁻⁷ /°C.

In this family of glasses the SiO₂ content is a function of the presenceof other oxides that are also difficult to melt, such as ZrO₂ and, ifapplicable, Al₂ O₃. Thus their maximum SiO₂ content must hot exceedapproximately 63%: above this, the melting of the vitrifiable mixtureand the refining of the glass require high temperatures, which causeaccelerated wear of the refractories of the furnaces. Furthermore, ithas been observed within the framework of this invention that anincrease in the SiO₂ content does not promote the raising of the strainpoint of the glass. Below 45% by weight of SiO₂, the stability of theglasses according to the invention is insufficient.

The zirconiferous glasses according to this invention, which are mosteasy to melt, the viscosities of which are best adapted for floating ofthe glass on a bath of molten metal, and which have the highest strainpoints, contain between 45 and 59% of SiO₂.

As for the aluminous glasses, it has been established that the meltingof the zirconiferous glasses remains within acceptable temperaturelimits provided that the sum of the oxides SiO₂, Al₂ O₃ and ZrO₂ remainsequal to or less than 70%. The term acceptable limits should beunderstood to mean that the temperature of the glass corresponding tologη=1.6 does not exceed approximately 1,630° C. and preferably 1,590°C.

In the zirconiferous glasses according to the invention, the sum of theAl₂ O₃ and ZrO₂ oxides is advantageously equal to or greater than 8%,and preferably between 8 and 22%. Their ZrO₂ content is advantageouslybetween 8 and 15%.

These zirconiferous glasses are distinguished, in particular, from thealuminous glasses according to the invention by the absence of boronbecause, in contrast to Al₂ O₃, the presence of even a high content ofZrO₂ in the glasses of the invention does not have the effect ofincreasing their viscosity at high temperatures.

The zirconiferous glasses according to the invention also have theadvantage of being well adapted to the techniques of melting associatedwith the procedure of floating the glass on a bath of molten metal. Infacts it has been found that these glasses result in a low corrosion ofthe refractories, of the AZS (alumina-zirconia-silica) type, commonlyused in this type of furnace. These glasses thus guarantee anoptimization of the useful life of the furnace.

The preferred compositions of zirconiferous glasses according to theinvention contain the constituents listed below in the followingproportions by weight:

    ______________________________________                                               SiO.sub.2                                                                           45 to 59%                                                               ZrO.sub.2                                                                           8 to 15%                                                                Al.sub.2 O.sub.3                                                                    0 to 10%                                                                Na.sub.2 O                                                                          4 to 10%                                                                K.sub.2 O                                                                           3.5 to 7%                                                               CaO   1 to 12%                                                                MgO   1 to 7%                                                          ______________________________________                                    

the sum of the contents of the oxides SiO₂, Al₂ O₃ and ZrO₂ remainingless than or equal to 70%, the sum of the contents of the alkalineoxides being equal to or greater than 10%, said compositions comprising,perhaps also, the oxides BaO and/or SrO in proportions such that:

    14%≦MgO+CaO+BaO+SrO≦22%

said glass compositions having a strain point equal to or higher than550° C. and a coefficient of thermal expansion (α₂₅°-300° C.) lyingbetween 82 and 95×10⁻⁷ /°C.

Generally speaking, the influence of the other oxides upon thesuitability of the glasses according to the invention to be melted andfloated on a metal bath, as well as upon their properties, is asfollows:

The oxides Na₂ O and K₂ O enable the fusion temperatures of the glassesaccording to the invention and their viscosities at high temperatures tobe maintained within the limits indicated above. To achieve this, thesum of the contents of these oxides remains equal to or higher thanapproximately 8%. By comparison with an ordinary silico-sodo-calcicglass the simultaneous presence of these two oxides in the glassesaccording to the invention, sometimes in closely adjacent proportions,enables their chemical resistance, more precisely their hydrolyticresistance, and also their resistivity to be considerably increased. Theincrease in the resistivity of the glasses is advantageous in certainapplications, more precisely when they are used as a substrate for coldcathode ray screens (field emission displays). In these screens, surfaceelectrical fields originate, which cause a localized concentration ofelectrons. This concentration may cause an undesirable migrationreaction of the alkalines if the resistivity of the glass is inadequate,as in the case of an ordinary silico-sodo-calcic glass.

The alkaline earth oxides introduced into the glasses according to theinvention have the overall effect of raising the strain point, and thisis the reason why the sum of their contents by weight must be at least11%. Above approximately 30%, the tendency of the glasses to devitrifycan increase in proportions that are incompatible with the floatprocedure on a metal bath. In order to maintain the devitrification ofthe glasses within acceptable limits, their contents by weight of CaOand MgO must not exceed, respectively, 13 and 8%. The MgO content is,preferably, equal to or less than 5%.

MgO, CaO and to a lesser extent SrO enable the strain point to beraised; BaO and SrO enable the chemical resistance of the glassesaccording to the invention and also their resistivity to be increased.BaO also has the effect of lowering the melting temperature and also theviscosity of the glasses at high temperatures.

The advantages offered by the glass compositions according to theinvention will be better appreciated from the examples summarized in theannexed tables 1 and 2.

Glass no. 1 corresponds to a conventional silico-sodo-calcic glasscomposition used for the production of a glass ribbon by the process offloat glass on a molten metal bath; glass no. 2 corresponds to a knownborosilicate glass. Glasses nos. 3 to 13 illustrate the glasscompositions according to the invention. The strain point T₁, thecoefficient of thermal expansion, the viscosities and the liquidustemperature as well as the hydrolytic resistance (DGG) and theresistivity were measured by methods well known to the person skilled inthe art.

As the examples demonstrate, the viscosity and liquidus characteristicsof the glasses according to the invention are sufficiently close tothose of the reference glass for them to be produced and transformedinto a ribbon under virtually the same conditions as the latter.

Thus, by the float glass technique, the glasses according to theinvention are produced in the form of a ribbon of strictly controlledthickness, which may vary from 0.5 m to 10 mm. Sheets are cut to thedesired format from said ribbon before being subjected to a thermaltreatment having the objective of stabilizing the dimensions of saidsheets. These sheets are then ready to serve as substrate which willaccept the deposition of different films and the thermal treatments thattheir fixing requires.

These sheets or plates, having been subjected to thermal toughening, maybe incorporated into insulating glazing panes or into laminated panes.These insulating panes are constituted of plates associated in pairs bymeans of a bonded intermediate profile and the technique for mountingthem in the frame that supports them is such that, when they are exposedto flames, the edge of the plate on the fire side is exposedinstantaneously or, in any case, after a brief delay, to the thermalradiation and to the flames themselves, which makes it possible to limitthe thermal stresses that commonly become established in a plate when itis heated more at its centre than at its edges. The association of athermal toughening of good quality and of the mounting assembly inquestion allows such a pane to remain in place for a sufficient time tosatisfy the standards in force.

The laminated panes are produced by the association of plates by meansof an intermediate plastics film; in general, the glass plates used arealso thermally toughened.

                  TABLE 1                                                         ______________________________________                                        No 1       No 2    No 3   No 4 No 5 No 6 No 7 No 8                            ______________________________________                                        SiO.sub.2                                                                             71,7   81,0    60,9 66,6 53.6 46.9 51,1 48,5                          Al.sub.2 O.sub.3                                                                      0,6    2,2     5,7  3,0  10,0 18,0 12,0 14,8                          ZrO.sub.2                        2,0       1,9  2,0                           B.sub.2 O.sub.3                                                                              13      3,5  3,4  2,2  3,5  1,7  2,3                           Na.sub.2 O                                                                            13,9   3,6     5,5  9,6  5,2  4,7  4,7  5,3                           K.sub.2 O      0,2     6,2  4,1  6,2  6,2  6,8  6,5                           MgO     4,1            4,2  6,7  4,2  3,0  3,7  3,8                           CaO     9,5            6,8  6,6  6,8  7,5  6,8  6,6                           SrO                    4,4       7,0  7,2  7,6  7,0                           BaO                    2,8       2,8  3,0  3,7  3,2                           T.sub.l (°C.)                                                                  507    510     540  531  580  579  577  582                           T(logη =                                                                          725    821                              830                           7,6) (°C.)                                                             α (× 10.sup.-7 /                                                          88,5   32      84,5 80,5 84,0 86,0 88,5 88,0                          °C.)                                                                   Log ρ                                                                             6,6                                     8,7                           (Ω·cm)                                                         (at 250° C.)                                                           D.G.G. (mg)                                                                           30                       7                                            T(logη =                                                                          1550   >1800   1566 1579 1584 1559 1554 1546                          1,6) (°C.)                                                             T(logη =                                                                          1085           1113 1119 1156 1160 1159 1162                          3,5) (°C.)                                                             T.sub.liquidus (°C.)                                                           1020           1060 1110 1120 1100 1120 1120                          ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        No 1      No 2    No 9   No 10*                                                                              No 11*                                                                              No 12*                                                                              No 13*                             ______________________________________                                        SiO.sub.2                                                                            71,7   81,0    54,6 52,0  53,05 52    52                               Al.sub.2 O.sub.3                                                                     0,6    2,2     3,0  4,0   3,25  2     5,0                              ZrO.sub.2             10,0 11,0  9,25  5     4                                B.sub.2 O.sub.3                                                                             13                                                              Na.sub.2 O                                                                           13,9   3,6     6,0  7,0   4,25  4,0   5,0                              K.sub.2 O     0,2     6,9  5,0   6,05  8,0   6,0                              MgO    4,1            4,2  7,0   2,05  4,0   5,0                              CaO    9,5            3,5  9,0   7,2   8,0   10,0                             SrO                   8,0  3,0   9,15  9,0   7,0                              BaO                   3,8  2,0   5,75  8,0   6,0                              T.sub.l (°C.)                                                                 507    510     606  600   612   574   575                              T(logη =                                                                         725    821                                                             7,6) (°C.)                                                             α (× 10.sup.-7 /                                                         88,5   32      81,5 84    81,5  93,5  91,3                             °C.)                                                                   Log ρ                                                                            6,6            9,7  9,65  10,7  11,3  10,7                             (Ω·cm)                                                         (at 250°                                                               C.)                                                                           D.G.G. 30                                                                     (mg)                                                                          T(logη =                                                                         1550   >1800   1554 1450  1539  1413  1415                             1,6) (°C.)                                                             T(logη =                                                                         1085           1192 1220  1172  1078  1072                             3,5) (°C.)                                                             T.sub.liquidus                                                                       1020                1360  1120                                         (°C.)                                                                  ______________________________________                                         *No. 10 to 13  Theoretical compositions                                  

We claim:
 1. A glass composition consisting essentially of the followingconstituents in the following proportions by weight:

    ______________________________________                                               SiO.sub.2     45 to 63%                                                       ZrO.sub.2    6.5 to 20%                                                       Al.sub.2 O.sub.3                                                                             0 to 18%                                                       Na.sub.2 O     4 to 12%                                                       K.sub.2 O    3.5 to 7%                                                        CaO            1 to 13%                                                       MgO            0 to 8%                                                 ______________________________________                                    

wherein the sum of SiO₂, ZrO₂ and Al₂ O is equal to or less than 70%,the sum of Na₂ O and K₂ O is equal to or greater than 8%, saidcomposition containing, optionally, the oxides BaO and/or SrO inproportions such that:

    11%≦MgO+CaO+BaO+SrO≦24%

said composition has a strain point equal to or greater thanapproximately 530° C. and a coefficient of thermal expansion (α25°-300°C.) between 80 and 95×10⁻⁷ /°C.
 2. The glass composition according toclaim 1, wherein the sum of Al₂ O₃ and ZrO₂ is equal to or greater than8%.
 3. The glass composition according to claim 2, wherein the sum ofAl₂ O₃ and ZrO₂ is between 8 and 22%.
 4. The glass composition accordingto claim 1, wherein the amount of ZrO₂ is from 8 to 15%.
 5. The glasscomposition according to claim 1, wherein the amount of SiO₂ is from 45to 59%.
 6. The glass composition according to claim 1, consistingessentially of the following constituents in the following proportionsby weight:

    ______________________________________                                               SiO.sub.2                                                                           45 to 59%                                                               ZrO.sub.2                                                                           8 to 15%                                                                Al.sub.2 O.sub.3                                                                    0 to 10%                                                                Na.sub.2 O                                                                          4 to 10%                                                                K.sub.2 O                                                                           3.5 to 7%                                                               CaO   1 to 12%                                                                MgO   1 to 7%                                                          ______________________________________                                    

wherein the sum of Na₂ O and K₂ O is equal to or greater than 10%, saidcomposition optionally containing also the oxides BaO and/or SrO inproportions such that:

    14%≦MgO+CaO+BaO+SrO≦22%

said composition has a strain point equal to or greater thanapproximately 550° C. and a coefficient of thermal expansion (α25°-300°C.) between 85 and 95×10⁻⁷ /°C.
 7. The glass composition according toclaim 1, wherein said glass composition has a viscosity corresponding tologη=1.6 at a temperature equal to or less than 1,630° C.
 8. The glasscomposition according to claim 1, wherein said glass composition has aviscosity composition to logη=1.6 at a temperature equal to or less than1,590° C.
 9. The glass composition according to claim 1, wherein saidglass composition has a viscosity corresponding to logη=3.5 at atemperature equal to or less than 1,220° C.
 10. The glass compositionaccording to claim 1, wherein said glass composition has a viscositycorresponding to logη=1.6 at a temperature equal to or less than 1,590°C.
 11. The glass composition according to claim 9, having a liquidustemperature less than or equal to the temperature corresponding to theviscosity logη=3.5.
 12. The glass composition of claim 1, in the form ofa glass sheet.